1. Field of the Invention
The present invention relates to an apparatus for recovering exhaust heat from an engine and particularly, an engine exhaust heat recovering apparatus for used as a power source in a cogeneration system.
2. Description of the Related Art
As a variety of attempts for protection of the global environment have been proposed, energy friendly systems such as cogeneration systems for recovering the generated heat for reuse are now focused in view of the efficient usage of energy. Such a cogeneration system has an exhaust gas released at a higher temperature than that of its engine. Therefore, in the cogeneration system, a heat recovering medium (typically an engine cooling water) is introduced into the cooling section of the engine to receive the heat from an exhaust gas and then transferred to the heat exchanger where it releases the heat (See JP Patent No. 2691372 and JP Patent Laid-open Publication (Heisei)8-4586).
FIG. 5 is a diagram showing changes in the temperature of the heat recovering medium (referred to as a heat transfer medium hereinafter) and the exhaust gas in a conventional waste heat recovering apparatus where the vertical axis represents the temperature and the horizontal axis represents flow direction of the heat transfer medium and the exhaust gas. The temperature of the heat transfer medium varies as is denoted by a characteristic curve Lm while the temperature of the exhaust gas in the exhaust gas heat exchanger is denoted by the curves Lga (in parallel-flow mode) and Lgb (in counter-flow mode). The directions of the exhaust gas flow in the parallel-flow mode and the counter-flow mode are expressed by pf and cf, respectively.
The heat transfer medium recovers heat from the engine while running through the engine cooling unit, hence increasing its temperature from p′ to q′. As the heat transfer medium passes through the exhaust gas heat exchanger, it recovers heat from the exhaust gas and its temperature is increased from q′ to r′. Simultaneously, the exhaust gas is deprived of its heat by the heat transfer medium decreases its temperature from g′ to approximately r′. As the exhaust gas heat exchanger is disposed at the downstream with the engine cooling unit disposed at the upstream of the circulation path of the heat transfer medium, the heat transfer medium is exposed to the exhaust gas of a higher temperature than that of the engine in the exhaust gas heat exchanger, hence increasing a temperature difference Δt before and after recovering the waste heat (cf. the temperature difference Δt being slightly greater in the counter-flow mode than in the parallel-flow mode)
The recovered thermal quantity is proportional to a product of the temperature difference Δt of the heat transfer medium between the entrance and the exit of the waste heat recovering apparatus and the flow of the heat transfer medium. Accordingly, the smaller the flow, the greater the temperature difference Δt (or the recovered thermal energy) of the heat transfer medium becomes as shown in FIG. 5. If the flow of the heat transfer medium is great, the temperature difference will be declined.
FIG. 6 illustrates a temperature change of the heat transfer medium of which the flow is greater than that shown in FIG. 5. The temperature of the heat transfer medium varies as is denoted by a curve Lm1 when its temperature level at the entrance of the exhaust gas heat exchanger is lower than the dew point W of the exhaust gas. It varies as is denoted by a curve Lm2 when the temperature of the heat transfer medium is higher than the dew point W. The temperature of the exhaust gas is changed as indicated with lines Lg1 and Lg2 in both the cases. For simplicity of the description, the two cases are implemented in the parallel-flow mode.
The heat transfer medium introduced to the engine with its temperature a″ lower than the dew point W of the exhaust gas recovers heat from the engine, then its temperature increases to c″. Furthermore, as the heat transfer medium is passed through the exhaust gas heat exchanger, its temperature is increased in two steps. When the exhaust gas is deprived of its heat by the heat transfer medium, its temperature rapidly drops down from g. On the other hand, the heat transfer medium recovers heat from the exhaust gas and its temperature increases up to b″ when the temperature of the exhaust gas drops down to the dew point W. As the exhaust gas reaches the dew point W, its contents (mainly water vapor contained in the exhaust gas) are condensed thus generating a condensation heat. The condensation heat is also absorbed by the heat transfer medium of which the temperature in turn rises up to f″. Finally, the thermal energy bringing about a temperature difference Δt1 can be recovered.
Alternatively, the heat transfer medium having a temperature p″ higher than the dew point W of the exhaust gas and being introduced to the engine recovers heat from the engine, then its temperature increases to q″. Furthermore, as the heat transfer medium is passed through the exhaust gas heat exchanger, its temperature is increased to r″. As a result of the heat recovering just mentioned, a temperature difference Δt2 can be recovered.
As clearly understood with the comparison between FIGS. 5 and 6, the conventional waste heat recovering apparatus when using a large amount of the heat transfer medium for heat energy recovery possibly limits the temperature difference of the heat transfer medium between before and after recovering the waste heat to a smaller level than with the use of a smaller amount of the heat transfer medium. It may be possible to increase the temperature of the heat transfer medium due to a heat of condensation of the contents of the exhaust gas when the initial temperature of the heat transfer medium is lower than the dew point W of the exhaust gas. However, the temperature of the heat transfer medium is duly increased by heat transfer from the engine cooling unit before the heat transfer medium arrives at the entrance of the exhaust gas heat exchanger. This causes the temperature of the heat transfer medium to hardly stay lower than the dew point W. It will hence take a significant length of time before the exhaust gas temperature drops down to the dew point W, thus rarely permitting the transfer of the condensation heat at a higher efficiency. If the temperature of the exhaust gas is quickly decreased to the dew point W, more portions of the condensation heat energy may be transferred to the heat transfer medium. This issue has not yet been overcome.